Math & Science ⇒ How can I place a dash on top of a variable?

[Ca29]

Hi guys,

1 - I am trying to put a trace over variable to symbolize that it is constant.

2 - And I'm also trying to put a point on top of the variable to symbolize that the variable is a rate of change.

Thank you in advance for your attention.

[Ijon Tichy]

\documentclass{article}

\begin{document}
Usual: $\mathrm{a}$ is a constant, not a variable.
Unusual: $\bar{a}$ is a constant, not a variable.

Usual: $\dot{a}$
\end{document}

BTW: Something is either a variable or a constant. You should neither call is constant variable nor variable constant,

[Ca29]

Thanks Ijon